During the current year conductance pathways in Necturus fundic and antral gastric mucosa have been quantitated. Gastric fundus had a transepithelial resistance of 2565 omega cm squared and the cellular component was 3258 omega cm squared while the paracellular component was 12055 omega cm squared. Antral mucosa from the same species has a transepithelial resistance of 1424 omega cm squared. The resistance of antral cells was 7338 omega cm squared and paracellular shunt resistance was 1973 omega cm squared. Thus in fundic mucosa the paracellular shunt constitutes only 20-25% of total epithelial conductance while in antral mucosa the paracellular shunt contributes 75% of epithelial conductance. Fundic paracellular shunt resistance was 6 times antral shunt conductance. Antral shunt selectivity was found to be pH dependent and became increasingly anion selective with decreasing pH. Carboxyl group reagents (carbodiimide) increased anion selectivity at all pH's. Initial studies using fluorescent probes (ANS and dansyl phosphatidyl ethanolamine) have indicated changes in membrane structure dependent on pH and have also suggested changes upon addition of aspirin to purified gastric membrane.